10 theories about missing Flight MH370.

The Australian Marine Safety Authority (right) is one of many organizations still searching for Flight MH370.Photo: EPA

Was lost Flight MH370 the victim of a midair heist of gold bullion? Has it landed but remains hidden? Or was it a terrorist hijacking?

Speculation grows as Australia leads the Indian Ocean search for clues.

Top 10 theories – What may have happened to Flight MH370

1. Fire

A fire may have broken out on the aircraft shortly after last contact was made with air traffic control. The pilot may have made a sharp left turn and attempted to return to a Malaysian airport.

Chris Goodfellow, a Canadian pilot with 20 years’ experience, believes Capt. Zaharie Ahmad Shah was taking a direct route to Palau Langkawi, a 13,000-foot (3,962-meter) airstrip with an approach over water and no real obstacle. If the plane had turned back toward Kuala Lumpur, it would have needed to cross a series of high ridges.

According to him, an electrical fire could explain MH370’s failure to communicate.

He said the flight crew was most likely overcome by smoke and the plane continued in the same direction until it ran out of fuel or fire destroyed the control surfaces and it crashed.

2. Decompression

A gradual decompression could also explain MH370’s disappearance.

Central Queensland University aviation expert Ron Bishop said a fire or some other accident could have led to a gradual decompression of the aircraft.

The pilots may have tried to return to Malaysia but passed out after turning the aircraft around and the plane continued flying until it ran out of fuel.

3. Mechanical failure

The plane may have suffered a mechanical failure that required the aircraft to turn back to Malaysia. However, the problem could have become so severe that the pilots lost control of the aircraft and the plane may have continued flying until it ran out of fuel or crashed.

4. Hijacking

A passenger may have gained access to the cockpit and hijacked the plane for a political, psychological or financial reason. If the passenger was armed, they might have threatened the pilots and forced them to fly off course.

An instructor at the Tianjiao Special Guard/Security Consultant bodyguard training camp teaches students about close-quarter combat aboard planes.Photo: Reuters

Otherwise, if the passenger was a trained pilot, they could have taken over the controls.

Malaysia Airlines has said it was “shocked” at a TV report that MH370 co-pilot Fariq Abdul Hamid had allowed two women into the cockpit on a flight in 2011.

5. Terror attack

The plane may have fallen victim to a terrorist attack.

The presence of two passengers traveling on stolen passports fueled early fears of a terror link.

Authorities now believe the two Iranian men were simply illegal migrants, but CIA Director John Brennan has said a terror attack has not been ruled out.

The search area covers a massive region — a northern corridor from the border of Kazakhstan and Turkmenistan through northern Thailand and a southern corridor from Indonesia to the southern Indian Ocean — with no potential target or destination pinpointed so far.

6. Pilot involvement

Malaysian Prime Minister Najib Razak said on Saturday evidence indicated that the plane’s communication systems were manually switched off and it was deliberately diverted.

This suggests that one or both of the pilots could have been involved in diverting the Beijing-bound flight and fly it toward the Indian Ocean.

Malaysian authorities have raided the homes of Zaharie and Fariq, searching for evidence to link them with its disappearance.

Zaharie was reportedly a fanatical supporter of Anwar Ibrahim who was incensed by his sodomy verdict.

7. Pilot suicide

While rare, there have been cases of pilots crashing planes to take their own lives.

In December 1997, a SilkAir Boeing 737 from Jakarta to Singapore plunged into a river in Indonesia with the loss of 104 passengers and crew. US investigators blamed pilot suicide.

A suicide bid “is possible, and if that’s the case, there might not be a lot of debris because the plane would have come down in relative structural integrity,” said Terence Fan, an aviation expert at Singapore Management University.

“The airplane is not meant to float and if the airplane sinks in the water, water will go inside because the door seals are not meant to seal water.”

However, nothing has emerged to suggest any serious psychological problems with either of the pilots who were flying MH370.

8. Plane landed but is hidden

The lack of debris and apparent absence of any data indicating impact has led to speculation that the plane may have landed safely and be hidden in a remote location.

However, the size of the Boeing 777 and the amount of space needed for it to land make it unlikely that this was the flight’s fate, said Greg Waldron, Asia managing editor at industry publication Flightglobal.

“The triple 7 is a very large aircraft that requires a long airport-size runway to land … it’s possible, but I think not probable.”

9. Cover-up

The apparent slowness to reveal key radar data has led to speculation that countries may know more about the plane’s likely whereabouts, but are unwilling to share due to a perceived security risk.

Jakarta-based independent aviation analyst Gerry Soejatman said that if the aircraft flew north across Asia, following one of two air corridors currently being searched, it would be remarkable if no one had detected it.

“It’s extremely difficult to comprehend that so many countries might have seen it and kept it under wraps.”

10. Theft

Some have speculated the Boeing 777 may have been stolen, either to be used in a later terror attack or because it was carrying a valuable cargo, such as gold.

Experts have also played down the likelihood the plane would have been taken for a terrorist attack, pointing out that there are thousands of business jets and cargo planes that could be captured and used much more easily.